Hibernate annotations

[jagrutkumar dalwadi]

I am using Hibernate 3. In that I am using
hibernate.hbm2ddl.auto=update
utility for generating the db from Pojo with annotations.

I am facing one odd behaviour which is as below:
1.

@Column(nullable = false, length = 40, unique = true)
	public String getInvoicingType() {
		return invoicingType;
	}

When I have put unique constraint on my getter method, it is creating Unique constraint in database.

2. However when I have put this annotation on my field, it is not creating unique constraint in database.

@Column(nullable = false, length = 40, unique = true)
private String invoicingType;

can anyone suggest regarding such behaviour?

Thanks